In recent years, steel sheets on the surface of which rustproof treatments are performed, in particular, galvanized steel sheets and galvannealed steel sheets which exhibit excellent rust-proofness, are used in such fields as automobiles, home appliances and building materials. On the other hand, from the point of view of enhancing the fuel efficiency of automobiles and the safety of automobiles in the event of crash, lightweight and strong bodies are pursued by increasing the strength of body materials and thereby allowing the thickness to be decreased. For this purpose, high-strength steel sheets are increasingly applied to automobiles.
In general, in production of galvanized steel sheets, thin steel sheets which are obtained by hot-rolling a steel slab or by further performing cold-rolling are used as base steel and the base steel sheets are subjected to recrystallization annealing in an annealing furnace on a CGL and thereafter hot galvanizing treatment. In the case of galvannealed steel sheets, the hot galvanization is followed by alloying treatment.
The addition of silicon and manganese is effective for increasing the strength of steel sheets. However, silicon and manganese are oxidized during continuous annealing even in a reductive N2+H2 gas atmosphere which does not cause the oxidation of iron (which reduces iron oxides), forming oxides of silicon and manganese on the outermost surface of the steel sheets. Such oxides of silicon and manganese cause a decrease in the wettability of the base steel sheets with respect to molten zinc during the coating treatment. Consequently, steel sheets containing silicon and/or manganese frequently suffer bare spots or, if not bare spots, poor coating adhesion.
Patent Literature 1 discloses a method in which galvanized steel sheets are produced using high-strength steel sheets that contain large amounts of silicon and manganese, as base steel. In the disclosed method, reducing annealing is performed after an oxide film is formed on the surface of the steel sheets. However, good coating adhesion cannot be obtained stably by the method of Patent Literature 1.
To solve this problem, Patent Literatures 2 to 8 disclose techniques directed to obtaining stable effects by specifying the oxidation rate or the amount of reduction, or by actually measuring the thickness of oxide films formed in the oxidation zone and then controlling the oxidation conditions or the reduction conditions based on the measurement results.
In Patent Literatures 9 to 11, the composition of gases such as O2, H2 and H2O in the atmosphere during the oxidation-reduction step is specified.